The present invention relates to rotary heat exchangers and more particularly to an improved attached tube cooler for cooling hot solid particulate material which has been thermally processed in a rotary kiln.
Attached tube coolers, sometimes referred to as planetary or satellite coolers, used in conjunction with rotary kilns for cooling hot particulate material discharged from the rotary kiln have been in use for many years. An attached tube cooler will include a plurality of circumferentially spaced apart cylindrical cooler tubes which are attached to the discharge end of a rotary kiln. Each cooler tube is mounted on the kiln so that its longitudinal axis is parallel to and and offset from the longitudinal axis of the kiln. The tubes may be mounted on the kiln so that the cooler discharge is downhill from the cooler inlet and kiln discharge so that material moves through the cooler by gravity as it tumbles within the cooler tube, or the tubes may be mounted so that the cooler discharge is uphill from the cooler inlet and kiln discharge. In the later instance, a helical conveyor is provided in the cooler tubes to advance material through the cooler as the material tumbles within the cooler tube during the cooler's rotation. The present invention is applicable to both types of installations, but the description will be directed to a cooler with an uphill cooler outlet.
Attached tube coolers operate on the principal of being attached to the discharge end of a rotary kiln for rotation with the kiln and for receiving at one end hot particulate material directly from the kiln outlet. Cooling gas such as ambient air is supplied to the other end of the cooler tube for countercurrent heat exchange contact with the hot material to thereby cool the hot material. As the material is cooled, the cooling gas is heated and returned to the kiln as preheated combustion air. As the cooler tubes rotate with the kiln about the longitudinal axis of the kiln, the material is tumbled within the cooler tube to achieve greater contact with the gas flowing through the cooler tube as well as advance the material from the inlet to the outlet.
Prior to the present invention, it was common practice to employ within rotary heat exchange apparatus various devices such as chains to improve the heat exchange contact between gas and solid.
Also prior to the present invention, it was common practice to employ lifters within the cooler tube in an effort to improve the contact between the particulate material and the cooling air. These lifters are creatively designed to pick up material from the bottom of the cooler tube. The material picked up by the lifters will fall from the lifters as the cooler moves through its rotational path about the axis of the kiln to thereby create a curtain of material within the cooler tube which is directly exposed to the gas flowing through the cooler tube. Typical examples of such apparatus are shown in U.S. Pat. Nos. 3,830,623 and 2,845,259.
Although the use of such lifters to produce a curtain of material within the cooler tube may create better contact between the cooling gas and the solid to be cooled, so much contact between the gas and the solid is created that a large quantity of the solid material will be entrained in the gas stream and returned to the kiln with the heated gas. This material which has been returned to the kiln must then be recycled back to the cooler thereby reducing the efficiency of the cooling apparatus as well as the kiln operation. This is particularly a problem when fine material such as alumina is being processed. It would, therefore, be desirable to reduce the amount of air-borne particles within the cooler tubes.
It has been found that efficient heat exchange can be achieved by increasing the surface area of metal exposed to the hot material so that the heat of the material is transferred to the shell of the cooler tube by conduction to thereby improve cooling of the material. More important, air flow through the cooler tube is used not only for cooling the material by direct contact between the air and material, but also to cool the metal which has been exposed to the hot material during a portion of the cooler's travel around the kiln axis, but the material is not exposed directly to the high velocity airflow during the balance of the cooler tube's travel around the kiln axis. By the present invention, additional heat exchange surface area is added to the cooler by using heat exchange fins which are designed so that they do not lift the material to fall in a curtain through the path of air flow through the heat exchanger. In the present invention, the heat exchange fins are "swept back" in the direction of rotation.
Some rotary heat exchangers prior to the present invention employed swept back fins, but conventionally these fins covered an air plenum and served to define an air passage from the plenum to the inside of the heat exchangers and prevent material from entering the plenum. Typical examples of such devices are shown in U.S. Pat. Nos. 2,483,738 and 2,581,756. Another example of swept back fins is shown in U.S. Pat. No. 3,357,382, but this device is used in an incinerator to insure complete combustion of ash and the fins of this patent are not designed for the purpose of improving heat exchange.
With many attached tube coolers, it is necessary to provide some means for advancing the material to be cooled from the material inlet to the material outlet. Such is the case when the cooler outlet is uphill from the cooler inlet. Attached tube coolers employing helical conveyors are known prior to the present invention as illustrated by U.S. Pat. No. 3,809,528 and many others, and it was even known to combine the lifters used to improve gas-solids contact and the conveyor as illustrated in U.S. Pat No. 3,556,495. The present invention is described as employing a helical conveyor within the cooler tube.